d later, but independently, and (fortunately
for him more publicly) by Leverrier.

None of these instances of the discovery of members of the solar system
resembled in method or details the discovery announced early in the year
1859. It was not amid the star-depths and in the darkness of night that
the new planet was looked for, but in broad day, and on the face of the
sun himself. It was not on the outskirts of the solar system that the
planet was supposed to be travelling, but within the orbit of Mercury,
hitherto regarded as of all planets the nearest to the sun. It was not
hoped that any calculation of the perturbations of other planets would
show the place of the stranger, though certain changes in the orbit of
Mercury seemed clearly enough to indicate the stranger's existence.

Early in 1860 Leverrier had announced that the position of Mercury's
path was not precisely in agreement with calculations based on the
adopted estimates of the masses of those planets which chiefly disturb
the motions of Mercury. The part of the path where Mercury is nearest to
the sun, and where, therefore, he travels fastest, had slightly shifted
from its calculated place. This part of the path was expected to move,
but it had moved more than was expected; and of course Mercury having
his region of swiftest motion somewhat differently placed than was
anticipated, himself moved somewhat differently.

Leverrier found that to explain this feature of Mercury's motion either
the mass of Venus must be regarded as one-tenth greater than had been
supposed, or some unknown cause must be regarded as affecting the motion
of Mercury. A planet as large as Mercury, about midway between Mercury
and the sun, would account for the observed disturbance; but Leverrier
rejected the belief that such a planet exists, simply because he could
not 'believe that it would be invisible during total eclipses of the
sun.' 'All difficulties disappear,' he added, 'if we admit, in place of
a single planet, small bodies circulating between Mercury and the sun.'
Considering their existence as not at all improbable, he advised
astronomers to watch for them.

It was on January 2, 1860, that Leverrier thus wrote. On December 22,
1859, a letter had been addressed by a M. Lescarbault of Orgeres to
Leverrier, through M. Vallee, hon. inspector-general of roads and
bridges, announcing that on March 26, 1859, about four in the afternoon,
Lescarbault had seen a round black spot